Bump-on-Trace (BOT) structures were used in flip chip packages, wherein metal bumps are bonded onto metal traces in package substrates directly, rather than bonded onto metal pads that have greater widths than the respective connecting metal traces. The BOT structures, compared with bonding structures bound to metal pads, help to reduce chip areas, and lower the manufacturing cost of the integrated circuit (IC) having BOT structures. In some applications, the BOT structures may achieve the same reliability as the conventional bond structures that are based on metal pads.
During the BOT manufacturing process, the metal bumps are soldered onto respective metal traces on the substrate by a reflow process. A reflowing process is a process in which solder is attached to a first conductive member, melted by heat, and then solidified. In some applications, the melted solder is caused to be in contact with both the first conductive member and a second conductive member, and then the solder connects the first and second conductive members after being solidified. Heating may be accomplished by passing the structure through a reflow oven or under a heating device, or by soldering individual joints with a hot air pencil.
During the reflowing process, the resulting packaging component needs to be cooled down from the hot reflow condition to room temperature. The drop in temperature would cause the metal traces and the substrate to contract. Because the metal traces and the substrate have different coefficient of thermal expansions (CTEs), the mismatch in the respective CTEs creates stress at the interface between the metal traces and the substrate. The CTE mismatch and the stress caused by the mismatch would result in the trace being dislocated from the substrate, which is also referred to as a phenomenon called “peeling.” This dislocation of the metal traces from the substrate, i.e., peeling, would cause the packaging device to fail prematurely.